1. Field of the invention
The present invention relates to an industrial process for preparing acetaldehyde diethyl acetal (herein called acetal) by reaction of acetaldehyde with ethanol in the presence of an acid catalyst, and in the presence of an entrainer having a boiling point of from 25.degree. to 75.degree. C.
2. Description of the Related Art
The preparation of acetal is known in the literature. The process described in Org. Synth. Coll. Vol. I, 1948, page 1, is a laboratory process which, because of its high consumption of chemicals and the associated costs, is out of the question for industrial implementation.
For higher-boiling aldehydes and ketones such as butyraldehyde, industrially useful catalytic acetalization processes are known in the literature, (see Weygand/Hilgetag, Org.-Chem., Experimentierkunst (1970), J. A. Barth-Verlag Leipzig, page 393). The acetalization process is an equilibrium process which follows the general scheme below: ##STR1##
In the acetalization of higher aldehydes and ketones, the water of reaction formed is removed by means of an entrainer which boils at a lower temperature than the aldehyde or the ketone, or by means of the sparingly water-soluble aldehyde or ketone itself. This removal of the water of reaction shifts the equilibrium of the reaction to the desired side providing more complete reactions, (i.e. k.sub.1 &gt;&gt;k.sub.-1). Using the catalytic reaction, these are the only known means for achieving high yields.
Unfortunately, this conventional mode of operation is not possible in the case of low-boiling acetaldehyde (bp. 20.2.degree. C.), since the starting material tends to distill out of the reactor and escapes reaction (i.e. removal of acetaldehyde makes the equilibrium shift such that k.sub.-1 &gt;&gt;k.sub.1). Acetaldehyde is also not suitable for removal of water, because it is miscible with water in any ratio. The conventional acetalization methods are thus basically not applicable to acetaldehyde.
DE-A-34 03 426 proposes an industrial acetalization process which uses solid acid catalysts and in which the workup of the reaction product is carried out by the addition of large amounts of water and large amounts of an extractant immiscible with water. The extractant takes up the acetal and the unreacted aldehyde. The water takes up the alcohol. This separation is very important here, because in the case of acetal this forms an azeotrope with ethanol (boiling point: 78.degree. C., acetal content: 24%), so that a distillation alone would not give a pure product.
However, this process has the disadvantage that it requires large amounts of water and extractant, the handling of which leads to uneconomically high costs. In addition, in this and in all other known acetalization processes, one only obtains, at best, the equilibrium state in the reaction between aldehyde and alcohol, i.e., a large part of the aldehyde always remains unreacted and has to be recovered or destroyed, thus incurring additional costs.
It is therefore desirable to have a simple process which allows the reaction of acetaldehyde beyond the equilibrium state and which uses an effective workup which does not require large amounts of auxiliaries. Furthermore, there is great interest in a process for reacting acetaldehyde with ethanol in the presence of a catalyst and simultaneously removing the water of reaction to achieve a high acetaldehyde conversion with little technical effort and without the use of expensive chemicals.